Projection objectives are used for example in the manufacture of electronic building elements or other micro-structured components. In the manufacturing process, a template which is also referred to as a mask or reticle is optically projected by means of the projection objective onto a substrate which carries a light-sensitive coating.
The trend towards ever higher packaging densities in electronic building elements and the miniaturization which also progresses in other areas have the consequence that structures are being produced with continually decreasing dimensions. Accordingly, more stringent requirements are imposed on the projection objective being used, wherein the interest is focused particularly on the achievable resolution.
The shorter the operating wavelength and the larger the numerical aperture of the projection objective, the smaller are the structures that can be resolved and that can thus still be imaged. In order to resolve the smallest possible structures, projection objectives are therefore designed for shorter and shorter operating wavelengths. However, these efforts are challenged by the increasing difficulty of finding materials which are sufficiently transparent at the operating wavelength and from which the optical elements of the projection objective can be made with the required accuracy and at a justifiable cost. A particularly high numerical aperture can be attained with an immersion fluid that has a refractive index greater than 1 and occupies the space between a last optical element of the projection objective and the substrate surface. The immersion fluid takes the place of the gas or gas mixture which otherwise fills this space and has a refractive index close to 1.
The known state of the art already includes projection objectives which can be operated selectively with an immersion fluid or with a gas in the space between the last optical element and the substrate surface. For example, a method is disclosed in DE 102 58 718 A1 which allows the projection objective to be adapted between an immersion configuration and a dry configuration so that it can be used selectively as an immersion objective or as a dry objective. In the final outcome, this possibility of combining the two operating modes promotes the use of the immersion technique in projection objectives, because projection objectives of this convertible type can first serve as a replacement for purely dry objectives and because the immersion technique can also be used on a trial basis, keeping the return to the conventional dry technique open.
Nevertheless, the use of the immersion technique still involves a considerable degree of cost and complexity. Regardless of whether a pure immersion objective or a combined immersion/dry objective is being used, the procedure for developing immersion objectives is still to select the most ideal immersion fluid possible and to design the projection objective to be compatible with the selected immersion fluid. However, this has the consequence that only “established” immersion fluids find application, because in view of the high development costs, there is a desire to eliminate potential risk factors such as for example a not previously proven immersion fluid.
To the extent that the foregoing relates to DE 102 58 718 A1, it refers only to some individual aspects that are of interest in the present context and does not represent a binding interpretation of the content of DE 102 58 718 A1.